1. Field of the Invention
The present invention relates to a semiconductor laser diode.
2. Related Background Art
In late years, a semiconductor laser diode is used for reading and writing means for an optical disk in high-density optical disk system. For this semiconductor laser diode, InGaAlP compound semiconductor laser diode in the 600 nm wavelength range has been developed, and optical disk system such as DVD has been realized. In the DVD, high output power of about 10 mW and 50 mW is required for a semiconductor laser diode for reading and writing respectively.
Recently, development of a semiconductor laser diode having shorter wavelength is proceeding in order to increase the record density. As such laser diode, attention is paid to a GaN compound laser diode made of InGaAlBN. This GaN compound laser diode is demonstrated to have a wavelength of about 400 nm, and to have a lifetime longer than several thousand hours under room-temperature continuous-wave(cw) operation. This GaN compound laser diode is supposed to be promising on the basis of reliability.
For optical disk applications of GaN compound laser diodes, control of the oscillation mode characteristics is indispensable. However, in conventional GaN compound laser diode, there are some problems that it is difficult to obtain fundamental oscillation.=More specifically, there are some problems regarding control and stabilization of transverse modes perpendicular to the junction plane in conventional GaN compound laser diodes. One is that it is difficult to grow thick AlGaN clad layers due to the generation of cracks. This is why the differences of lattice constant and thermal expansion coefficient between the clad layer and other layers/substrate are large. Another is that the refractive index of GaN is higher than that of AlGaN, and so the mode field has a high optical confinement factor in the p/n-type GaN contact layers. As a result, the oscillating waveguide mode becomes a high-order transverse mode. Furthermore, the effective refractive index of the oscillating mode is lower than that of the GaN contact layers. Therefore, the resultant oscillating mode is predicted to behave as an anti-guide mode. So, in conventional GaN compound laser diode, it is difficult to obtain single-peak far-field pattern perpendicular to the junction plane. However, in laser diodes, single-peak far-field pattern is important because laser beam is focused on a minute spot when used. So, the conventional GaN compound laser diode can't be used stably until high output power, and threshold current density for the lasing increases with a decreasing optical confinement factor in such anti-guide structures.
On the other hand, if clad layer is thickened until the thickness required for fundamental transverse mode oscillation, there occurs a problem that lots of crack or crystal defects are generated. When the cracks or crystal defects are generated, laser oscillation becomes unstable, because leak current increases and the diode becomes liable to generate heat.